1. This invention relates to the field of metal casting, in particular to crucibles and molds for casting of reactive metal alloys such as those of titanium and aluminum.
2. Recently, there has been heightened interest in alloys of the titanium-aluminum system, most particularly those generally of the Ti.sub.3 Al (alpha-2) type and the TiAl (gamma) type. These alloys have the potential for serving in aircraft at higher temperatures than current titanium alloys and have much lower densities than the presently used nickel and iron based alloys. Experimental studies have shown that the titanium aluminum alloys present problems in melting and casting, insofar as reaction with materials heretofore known for containing titanium and aluminum. The alloys melt in the range of about 1450.degree. to 1650.degree. C.; for casting several hundred degrees of super heat is often desired. Thus, they tend to present melting and casting problems analagous to titanium, rather than aluminum based alloys.
As there has been little experience with casting titanium aluminum alloys, the prior art is only related to alloys comprised mostly of either titanium or aluminum. Both of these alloy systems have presented difficulty insofar as melting crucibles are concerned. Titanium alloys in particular have presented problems insofar as expendable molds are concerned.
In the melting of titanium based alloys, only water cooled copper crucibles have been found to be commercially useful. The melting point and reactivity of the molten metal causes container degradation and contamination of the casting with virtually all common refractories. Studies, such as reported by Garfinkle et al., in Transactions of the American Society for Metals, Vol. 58, pages 520-530 (1965), indicate the reactivity of molten titanium with various carbides, borides and silicides. Garfinkle et al. found cerium sulfide to have the greatest resistance, but dissolution was still said to be significant. Undoubtedly, certain laboratory chemicals may be resistant to titanium. But for commercial success, a container material must additionally have a satisfactory cost and availability and be formable into desired shapes. None has met all these criteria heretofore.
In conventional investment casting, the time of exposure of the mold material to molten alloy is relatively limited, compared to the crucible used for melting. Nonetheless, mold materials for casting titanium alloys still present a problem. When the mold materials usable with iron and nickel base alloys, such as metal oxides of silicon, zirconium and aluminum, are used for casting titanium alloys it is found that there is unacceptable interaction and introduction of debilitating oxygen into the casting. Molds of rammed graphite or metal oxide molds lined with graphite are usable for titanium alloys but excess carbon is found in an embrittled casting surface. Katz et al. U.S. Pat. No. 3,180,632 describes the use of a metal oxide such as yttria to coat a graphite mold and reduce interaction. Monolithic graphite containers present limitations on the types of shapes which can be formed; graphite-containing molds cannot be fired in conventional furnaces with oxidizing atmospheres. Molds with refractory metal linings, such as metal oxide molds having tungsten powder linings, and described in Brown et al. U.S. Pat. No. 3,537,949, present cost and manufacturing impediments. Other prior patent art on the foregoing types of molds is recited in Basche U.S. Pat. No. 4,135,030.
Compared to titanium, aluminum melting and casting is somewhat easier. Although the metal is quite reactive and reduces its own otherwise stable oxide, aluminum base alloys on the whole have considerably lower melting points than titanium alloys. Clay bonded silicon carbide and certain oxide materials are found to be suitable. But aluminum technology does not provide any useful materials for alloys containing substantial titanium, including titanium aluminum alloys, probably because of the higher melting point and reactivity of such alloys.
Thus, there is a need for improved materials for melting and casting of titanium aluminum alloys and other similarly reactive materials. An improved container material will be either unreactive, or have products of reaction which are not deleterious to the alloy, and will have a cost and availability which will make it commercially feasible.